A thermo-shrinkable film is used as packing material or labels for coating, binding or wrapping various containers including bottles, cans or the like, and longish objects including pipes, rods and so on, and is composed mainly of polyester.
A thermo-shrinkable film is employed for shrink (accumulative) packaging, shrink labeling and cap sealing various containers including, for example, polyethyleneterephthalte (PET) containers, polyethylene containers, glass containers and so on, using heat shrinkability thereof.
In order to manufacture a label or the like, polymer material is continuously melted and extruded, thus preparing an unstretched film, which is then stretched to give a thermo-shrinkable film roll. The film in the roll is unwound, slit into a desired width, and then rewound into the roll. Subsequently, various characters and figures, including a product name and the like, are printed on the resulting film. After the completion of printing, the film is folded and bonded along both edges by means of, for example, solvent adhesion, to give a tubular film (in a tubing process). As such, there are cases where the sequence of the slitting process and the printing process is reversed. The tubular film thus obtained may be rewound into another roll and then unwound in subsequent procedures so that it is cut into tubular labels of a desired length, which are then further converted into bags through bonding along an edge of one opening thereof.
Subsequently, a container is clothed in the label or bag, passed, for example on a belt conveyer, through a shrinking tunnel (steam tunnel) wherein heated steam is blown for thermal shrinkage or a shrinking tunnel (hot-air tunnel) wherein hot air is blown, to give a final product (a labeled container) having the label or bag tightly attached thereto.
In the manufacture of the product using the thermo-shrinkable film, subsequent procedures after the formation of the film, for example, the printing process, the process of passing the container clothed in the label or bag through the shrinking tunnel, etc., are accompanied by a predetermined heat. In particular, the shrinking process which is finally performed is accompanied by heat regardless of the heating means such as steam or hot air, and thus may incur damage to the label or result in a poorly printed state and may also change the properties of the film. Thereby, the label may have a weaker resistance to impact, resulting in lowered product reliability.
During the shrinking process using the steam tunnel or the hot-air tunnel, steam or hot air is blown from the inlet of the tunnel to the outlet of the tunnel, but both ends and the center of the tunnel are not uniform in temperature. In this way, when the temperature in the tunnel is not uniform, the degree of shrinkage of the label may vary slightly.
After the shrinking process using the shrinking tunnel or the hot-air tunnel, the label should be uniformly shrunk so as to be tightly attached to the container. Actually, however, the label may be warped or the shape thereof may be lost.
This phenomenon occurs because the label or the like is not uniformly shrunk for a predetermined period of time at a given temperature. The difference in the degree of shrinkage, namely, the non-uniformity of shrinkage, causes a defective outer appearance of the label, and in severe cases, a poorly printed state may result.
The non-uniformity of shrinkage may retard the shrinking process and may reduce the productivity.